Remove null=True for DNSDomain and DNSRecords

[peterbppb]

dns_ips slack module assumes domains and records cannot be null so it tries to strip them here: https://github.com/surface-security/surface/blob/main/surface/dns_ips/slack.py#L74-83

We should either remove null=True to avoid this issue or filter for that in the slack processor.

I think removing null=True from these model fields makes sense as I do not think we have any need for records/domains without a name.

[gsilvapt]

I'm in favour of removing these attributes in both models, they are not appropriate for their data types.
When releasing this, we might need to add a note for all forks take into account this change (they will need to provide a default, most likely).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.27%. Comparing base (8e4f55f) to head (f994e3a).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #133      +/-   ##
==========================================
+ Coverage   77.25%   77.27%   +0.02%     
==========================================
  Files          84       85       +1     
  Lines        4022     4026       +4     
==========================================
+ Hits         3107     3111       +4     
  Misses        915      915              

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